Platelets are anucleate bone marrow-derived blood cells that protect injured mammals from blood loss by adhering to sites of vascular injury and by promoting the formation of plasma fibrin clots. Humans depleted of circulating platelets by bone marrow failure suffer from life threatening spontaneous bleeding, and less severe deficiencies of platelets contribute to bleeding complications following trauma or surgery.
A reduction in the number of circulating platelets to below ˜70,000 per μL reportedly results in a prolongation of a standardized cutaneous bleeding time test, and the bleeding interval prolongs, extrapolating to near infinity as the platelet count falls to zero. Patients with platelet counts of less than 20,000 per μL are thought to be highly susceptible to spontaneous hemorrhage from mucosal surfaces, especially when the thrombocytopenia is caused by bone marrow failure and when the affected patients are ravaged with sepsis or other insults. The platelet deficiencies associated with bone marrow disorders such as aplastic anemia, acute and chronic leukemias, metastatic cancer but especially resulting from cancer treatment with ionizing radiation and chemotherapy represent a major public health problem. Thrombocytopenia associated with major surgery, injury and sepsis also eventuates in administration of significant numbers of platelet transfusions.
A major advance in medical care half a century ago was the development of platelet transfusions to correct such platelet deficiencies, and over 9 million platelet transfusions took place in the United States alone in 1999 (Jacobs et al., 2001). Platelets, however, unlike all other transplantable tissues, do not tolerate refrigeration, because they disappear rapidly from the circulation of recipients if subjected to even very short periods of chilling, and the cooling effect that shortens platelet survival is irreversible (Becker et al., 1973; Berger et al., 1998).
The resulting need to keep these cells at room temperature prior to transfusion has imposed a unique set of costly and complex logistical requirements for platelet storage. Because platelets are actively metabolic at room temperature, they require constant agitation in porous containers to allow for release of evolved CO2 to prevent the toxic consequences of metabolic acidosis. Room temperature storage conditions result in macromolecular degradation and reduced hemostatic functions of platelets, a set of defects known as “the storage lesion” (Chemoff and Snyder, 1992). But the major problem with room-temperature storage, leading to its short (5-day) limitation, is the higher risk of bacterial infection. Bacterial contamination of blood components is currently the most frequent infectious complication of blood component use, exceeding by far that of viral agents (Engelfriet et al., 2000). In the USA, 3000-4500 cases yearly of bacterial sepsis occur because of bacterially contaminated blood components (Yomtovian et al., 1993).
The mechanism underlying the unique irreversible cold intolerance of platelets has been a mystery as has its physiological significance. Circulating platelets are smooth-surfaced discs that convert to complex shapes as they react to vascular injury. Over 40 years ago investigators noted that discoid platelets also change shape at refrigeration temperatures (Zucker and Borrelli, 1954). Subsequent evidence that a discoid shape was the best predictor of viability for platelets stored at room temperature (Schlichter and Harker, 1976) led to the conclusion that the cold-induced shape change per se was responsible for the rapid clearance of chilled platelets. Presumably irregularly-shaped platelets deformed by cooling became entrapped in the microcirculation.
Based on our studies linking signaling to the mechanisms leading to platelet shape changes induced by ligands (Hartwig et al., 1995), we predicted that chilling, by inhibiting calcium extrusion, could elevate calcium levels to a degree consistent with the activation of the protein gelsolin, which severs actin filaments and caps barbed ends of actin filaments. We also reasoned that a membrane lipid phase transition at low temperatures would cluster phosphoinositides. Phosphoinositide clustering uncaps actin filament barbed ends (Janmey and Stossel, 1989) to create nucleation sites for filament elongation. We produced experimental evidence for both mechanisms, documenting gelsolin activation, actin filament barbed end uncapping, and actin assembly in cooled platelets (Hoffmeister et al., 2001; Winokur and Hartwig, 1995). Others have reported spectroscopic changes in chilled platelets consistent with a membrane phase transition (Tablin et al., 1996). This information suggested a method for preserving the discoid shape of chilled platelets, using a cell-permeable calcium chelator to inhibit the calcium rise and cytochalasin B to prevent barbed end actin assembly. Although addition of these agents retained platelets in a discoid shape at 4° C. (Winokur and Hartwig, 1995), such platelets also clear rapidly from the circulation, as we report here. Therefore, the problem of the rapid clearance of chilled platelets remains, and methods of increasing circulation time as well as storage time for platelets are needed.